What is the name of the fifth book of the New Testament?

The fifth book of the New Testament is Acts of the Apostles.

In early Christianity, 'Acts' was a literary genre applied to a whole range of supposed biographical/historical books, but Acts of the Apostles was the only one of this genre considered worthy of inclusion in the New Testament. The title might imply that it covers the lives of all or most of the apostles, whereas it is largely a record, and perhaps a comparison, of Peter and Paul. Nevertheless, it is probably the one book of Acts that does cover the two most important apostles, with some brief mentions of other apostles. On that basis, this book has the best claim to its title, although it is not necessarily a historical account.